Cyanate ester resins, developed during the 1980s, have joined epoxy resins and bismaleimide resins as the third major class of thermosetting resins. Polycyanurates or cross-linked cyanate resins are prepared by the cyclopolymerization of aromatic cyanate esters. These cyanate esters are bisphenol derivatives containing a plurality of cyanate functional groups. When heated, the cyanate functionality undergoes exothermic trimerization to form substituted triazine rings. Subsequent curing produces the thermosetting resin and forms three-dimensional networks of oxygen-linked triazine rings and bisphenol units, termed polycyanurates. Because no leaving groups or volatile byproducts are formed during cure, the cyclotrimerization curing reaction is classified as addition polymerization.
The formation of substituted triazine rings from the cyclic addition of three cyanate groups appears to proceed via a series of bimolecular collisions in step-growth fashion (Shimp, PMSE Preprints, ACS Meeting, New York, pp. 107-13 (4/86); Bauer et al., Acta Polymerica, 37:715-19 (1986)). This mechanism is facilitated by the transient stability of active hydrogen addition compounds and proton transfer from the imidocarbonate intermediate to subsequently colliding cyanates. The role of soluble transition metal compounds is primarily coordination, gathering cyanate groups in proximity to form a ring.
Trimerization rates of uncatalyzed liquid or molten dicyanates are impractically slow, being largely a function of the concentration of active hydrogen impurities, including moisture. Traditional catalysts are transition metal carboxylates dissolved in organic solvents (e.g., methyl ethyl ketone). Efficient curing of high purity monomers and prepolymers requires the incorporation of nonvolatile hydroxyl compounds, while safe catalysis of hot melt formulations requires enhanced solubility of metal carboxylates and chelates. Both needs have been met by predissolving the coordination metal compound in a nonvolatile liquid alkylphenol (e.g., nonylphenol, Shimp, U.S. Pat. No. 4,604,452 (1986), Shimp, U.S. Pat. No. 4,785,075 (1988)).
Cyanate esters will form addition compounds with phenols, alcohols, amines, imidazoles, and most other labile hydrogen compounds upon heating or base catalysis (Grigat et al., Angew. Chem. (Int'l Ed.), 6:206 (1967)). Analogous to blocked isocyanates, these cyanate adducts are thermally reversible, favoring the unblocked state at temperatures in the 150-200.degree. C. range and above. These addition products are primarily of importance as catalytic intermediates in ring formation.
The addition of water to the cyanate group forms an imidocarbonic acid intermediate which rearranges to the more stable carbamate structure. Carbamates decompose with evolution of carbon dioxide gas at temperatures in the vicinity of 200.degree. C. and above. In contrast to isocyanates, which react nearly instantly with water at room temperature (and over a period of hours with alcohols), uncatalyzed cyanates are stable for months in blends containing water, alcohols, and phenols. Uncatalyzed o-alkylated cyanates (e.g., 2,6-dialkyl cyanates) are essentially non-reactive with these active hydrogen sources. Catalysts which accelerate the addition of hydroxyl compounds (including water) to cyanates include tertiary amines, most coordination metal cure catalysts, and acids.
For these reasons, maximum hydrolytic and thermal stabilities are insured when absorbed moisture is minimized by proper storage of resins and prepregs or, if present, removed via vacuum molding procedures.
Cyanate esters function as curing agents for epoxide resins through a combination of oxazoline ring formation (co-reaction) and catalysis of epoxide homopolymerization (Shimp et al., 33rd Int'l SAMPE Symp. and Exhib., Anaheim, CA (3/7-10/88). Essentially, cyclotrimerization proceeds initially and the cyanurate rings which form function as nucleophilic catalysts for both oxazoline formation and polyetherification reactions. As little as 35% by weight cyanate ester monomer or prepolymer can convert equivalent excesses of epoxide. Cyclotrimerization catalysts also accelerate epoxide consumption in such hybrid systems.
Cyanate esters are currently employed in rapidly curing adhesive compositions used to bond semiconductor devices or chips, also known as dice, to carrier substrates. Such adhesive compositions may include, in addition to the cyanate ester, thermally and/or electrically conductive filler and a curing catalyst dissolved in alkylphenol.
One of the most outstanding characteristics of cyanate ester homopolymers is their low dielectric loss properties. While chemical compositions containing cyanate ester homopolymers include appreciable percentages of electronegative elements (about 10% oxygen and 10% nitrogen for many common polymers), their symmetrical arrangement around electropositive carbon atoms creates balanced dipoles of short moments which store surprisingly little electromagnetic energy. Dielectric constants decrease slightly when the frequency is increased into the gigahertz (10.sup.9 Hz) range. The multivalent transition metal cations and bulky carboxylate anions used as cure catalysts generally make no measurable contribution to energy storage (Dk) or to dissipation of stored electromagnetic field energy as leakage currents (Df).
Cyanate ester adhesive compositions have eliminated many of the deficiencies inherent in epoxy adhesives and polyimide adhesives, such as low glass transition temperature, high degree of ionic contamination, retention of solvent, and lengthy cure. However, presently available cyanate ester-containing attach paste compositions exhibit some deficiency with respect to homogeneity, i.e., such pastes have a tendency to become nonhomogeneous at ambient temperatures. Accordingly, there still remains room for improvement of die-attach pastes containing electrically conductive filler and polycyanate ester monomer in a variety of ways, e.g., in assuring the stability of paste homogeneity, in extending the pot life of such die-attach pastes, in reducing the cost of preparation, as well as the ease of preparation, by avoiding the use of potentially detrimental components (e.g., alkylphenols are acidic species, which are potentially corrosive), by avoiding the use of volatile components (which upon cure may vaporize, resulting in reduced wire bondability due to contamination of bond pads, and/or which may lead to the creation of voids in the cured resin), by having sharper, more rapid (i.e., snappier) curing profiles (thus improving cure cycle time), and the like.